Devices utilizing improved linbo&#39; holographic medium

ABSTRACT

Volume holographic devices utilize lithium niobate, LiNbO3, containing added iron ions. The added ions provide for improved resolution and contrast. Utilization is made of the improved material in devices designed for multiple image recording and readout. In a preferred embodiment, selection, as among images, is determined by the angle of incidence of the interrogating beam.

United States Patent Glass et al.

[54] DEVICES UTILIZING IMPROVED LINBO HOLOGRAPI-IIC M EDIUM [72]lnventors: Alastair Malcolm Glass, Millington; George Earl Peterson,Short Hills,

both of NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated, Murray Hill,NJ.

[22] Filed: July 21,1971

2| Appl. No.: 164,506

[52] 0.8. CI ..350/3.5, 340/173 [51] Int. Cl. ..G02b 27/22 [58] Field ofSearch ..350/3.5, 147; 340/173 [56] References Cited UNITED STATESPATENTS 3,544,189 12/1970 Chenetal. ..350/3.5

COHERENT LIGHT SOURCE OOLLIMATlNG LENS [451 Nov. 21, 1972 OTHERPUBLICATIONS Ashkin et aL, 9 Applied Physics Letters 72 (7/1966) Chen etal., 13 Applied Physics Letters, 223, (10/1968) Firester; 10 .1. AppliedPhysics 4842 (l 1/1969) Amodel, 18 Applied Physics Letters 22 (1/197 1)Amodel, l8 Applied Physics Letters 540, (6/1971) Primary Examiner-DavidSchonberg Assistant Examiner-Robert L. Sherman Attorney-R. .l. Guentheret a1.

[5 7] ABSTRACT Volume holographic devices utilize lithium niobate, LiNb0containing added iron ions. The added ions provide for improvedresolution and contrast. Utilization is made of the improved material indevices designed for multiple image recording and readout. ln apreferred embodiment, selection, as among images, is determined by theangle of incidence of the interrogating beam.

9 Claims, 1 Drawing Figure MODIFIED L|Nb03 RECORDING MEDlUM DETECTORARRAY ANGLLAR 9 DATA DEFLECTOR OUT E DEME ozfimoomm OnZ J QmEOOEmokomJmmo momDOm FIQ] .rzwmmIOo A. M. GLASS WWWORS a. 5 PETERSON 8,217?ATTORNEV DEVICES UTILIZING IMPROVED LINIIO 'HOLOGRAPI-IIC MEDIUMBACKGROUND OF THE INVENTION 1. Field of the Invention The invention isconcerned with volume holographic media and devices. Bit density isenhanced by multiple image recording.

2. Description of the Prior Art Optical memory systems including bothdigital and analogous arrangements, the latter exemplified by holographydescribed in texts on optics became practically realizable with thecommercial availability of the laser about a decade ago. This type ofrecording depends upon the developed interference pattern of two or morebeams of electromagnetic radiation, one of which is designated areference beam while the other/s, sometimes containing intelligence, maybe designated signal beam/s. Reconstruction of recorded interferencepatterns is accomplished by an interrogating beam ordinarily madeincident on the recording medium at the Bragg angle.

Two-dimensional holographic arrangements depending on this principlehave been in widespread experimental use for some years. A number ofsuitable media, many of which operate on a photographic principle, havebeen utilized.

The concept of volume holography is dependent on the inherentcharacteristic of spacial redundancy, i.e., the fact that repeating setsof interference fringes contain the same information. Multiple imagingtakes advantage of the use of different angles of incidence of one ormore of the interfering waves thereby resulting in sets of fringes whichmay be selectively accessed. Selective access is accomplished byutilization of an interrogating beam at such angle of incidence that theBragg requirement is met. The fact that fringes of different orderimages may supplement or even result in occasional erasure is of noconsequence in a medium sufficiently thick to provide for the necessaryredundancy.

The concept of volume holography has been an extremely enticing one.Capability of such systems based on presently available optics suggeststhe possibility of random access memories containing from l" to [0" bitsof information in a reasonable volume (a cubic centimeter or less). Infact, at this time, there would appear to be no other proposed systemsinherently capable of such capacity.

It is recognized that the failure of holography to reach commercialfruition arises from deficiencies in present recording media.

Search for an appropriate holographic medium has included investigationof a variety of materials. Traditional gelatins, photographic emulsions,are generally useful two-dimensional media although they are notgenerally easily erasable. Transparency is generally sufficientlylimited to make the development of a reasonably thick medium unlikely.

Photochromic materials such as single crystalline KBr have been proposedfor this use. The phenomena relies on a change of absorption and/orindex of refraction on illumination. Induced change is erasable usuallyby thermal means. Limitations largely involve low diffraction efficiencyand, in the case of organic photochromics, chemical instability.

Magnetic materials depend upon a form of Curie point writing. A magneticfield is imposed across a medium. Illumination results in local heatingsufficient to reduce the coercivity of the medium and so enable theapplied field to produce reversal in magnetization direction. Readout isaccomplished, for example, by use of a detector sensitive to Faradayrotation of an accessing beam. Such systems are recommended by theirinherently high resolution, good stability, and easy thermal erasure.The recording process, however, usually requires high peak power.

A system that has certain inherent advantages utilizes crystallineferroelectrics. Their suggested use first arose from the observation ofoptically-induced damage." Such material, when utilized in opticalcircuit elements, was found to develop scattering centers attributableto the development of local refractive index inhomogeneities. It hasbeen recognized that at least one such material, LiNbO,, is capable ofhigh resolution. See U. S. Pat. No. 3,544,189. Erasure may be easilyaccomplished in bulk, by thermal means, or selectively, by opticalmeans. High diffraction efficiency is to be expected althoughoptimization has awaited identification of the mechanism responsible forthe development of index inhomogeneities.

SUMMARY OF THE INVENTION LiNbO, as a memory medium for use, for example,as a digital deflector recording medium or as a volume holographicmedium is improved by incorporation of small amounts of iron ions underconditions such that there is a reasonably even distribution of suchions in the divalent and trivalent states. Addition of iron is such asto bring the total content to a minimum level of ppm in units of cationcontent (100 ppm is approximately the same as 0.01 mole percent Fe,0,).For usual holographic media thicknesses, a maximum content of Fe ofabout 500 ppm is prescribed. The total iron content may reach a maximumof about 1,000 ppm, for even distribution of divalent and trivalent ionsor higher values where the trivalent form is prevalent. In more preciseterms the limits may be expressed as 50, 250 and 500 ppm Fe" and amountsof Fe" which desirably at least equal but may exceed such amounts. SinceFe has little effect on transparency the excess may be appreciable,e.g., 100 percent or greater.

Introduction of iron content may be in initial ingredients during growthor subsequently. Other processing steps are generally conventional butare designed so as to enhance the Fe'VI e ratio and to provide theappropriate distribution of ionic content, or at least so as to leavesuch ratio unimpaired. For example, ferroelectric poling required toproduce single domain material is maintained for as short a period aspossible to prevent ion clumping.

A device embodiment of the invention involves means for varying theangle of at least one of the write or read beams so as to provide formultiple imaging. One such preferred embodiment involves use of anacoustooptic element for accomplishing this end.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic view inperspective depicting an illustrative system designed for volumeholographic recording and interrogation.

DETAILED DESCRIPTION 1. The Drawing The depicted system utilizes acoherent light source 1 yielding coherent beam 2, spacial deflectors 3and 4, collimating lens 5, angular deflector 6, modified LiN- b0,recording medium 7 and detector 8. As depicted, the system is describedin terms of interrogation. For writing information, means shown areprovided for interfering laser beam 2 or a portion thereof with one ormore information-containing beams. The depicted system provides for aphase hologram formed in a thick medium (a medium of sufficientthickness to contain a repeating number of interference fringe sets soas to statistically compensate for destruction by members of othersets). Recording a hundred images within a volume defined by a commonsurface requires a thickness of approximately 0.25 millimeters undergeneral operating conditions. This thickness is accordingly considered apreferred minimum for these purposes.

A phase hologram formed in the transparent holographic medium 7 may beconsidered a three-dimensional diffraction grating which diffracts alight beam in the same way that a crystal lattice diffracts amonochromic beam of x-rays. There are well-defined Bragg angles ofincidence at which diffraction is maximized. Away from these anglesthere is no coherent diffraction. Several holograms may be superimposedon the same volume as the recording medium and any one may beselectively interrogated by appropriate choice of the angle ofincidence.

Coherent light source 1 may be a laser or more involved apparatusinvolving ancillary elements (second harmonic generators, etc.) yieldingradiation having a fundamental wavelength of less than about 0.65micrometers. This wavelength minimum is prescribed since it correspondswith a minimum quantum energy for effectively producing the refractiveindex change upon which the recording mode depends. This is described insome greater detail in a subsequent section herein. in the interrogationmode, source 1 may be operated at a somewhat longer wavelength. Suchoperation is desirable in that it permits long exposure without dangerof erasure but complicated operation in that it requires an adjustmentin the angle of incidence from that utilized in the recording mode tosatisfy the Bragg angle condition. Spacial deflectors 3 and 4, the firstproducing X deflection and the second producing Y deflection, mayoperate on any suitable principle, for example, acoustooptic,electrooptic, magnetooptic, etc. Means for producing such deflection arenot shown and may include acoustic generators and terminators, electrodestructures, means for applying magnetic or electric fields, energysources, etc. For detailed description of appropriate apparata, see Vol.46, Bell System Technical Journal, p. 957 (1967) and Vol. 6, Journal ofQuantum Electronics p, 223 (1970).

The function of x,y spacial deflectors 3 and 4 is to provide the firstlevel of spacial addressing. These elements may be considered asdividing the recording medium 7 into a grid corresponding withindividual recording regions. Collimating lens 5 serves to focus thebeam and thereby produce a well-defined demarltation between adjacentregions. Element 6 is a large aperture angular deflector which selectsthe desired recording or interrogation angle. Element 6 may also operateon any deflection principle involving interaction of light with, forexample, acoustic, magnetic, or electric energy. Since deflector 6 is injuxtaposition to LiNbO, element 7, the angle of incidence of the beam inthe storage medium has little effect on position, i.e., essentially theentirety of the designated region within the grid defined by deflectorelements 3 and 4 may be utilized. The depicted arrangement includes adetector array 8, as defined, to detect the information yielded duringthe interrogation of recording medium 7. Such an array may be composed,for example, of silicon photodiodes.

It has been shown that at least superimposed holographic images can berecorded and subsequently read out by approximate angle selectionwithout appreciable cross talk. Since present technology permitsattainment of a grid of about 10' regions on a feasible area (of theorder of 1 square centimeter), a system such as that depicted in theFIGURE is, in principle, capable of providing 10 information bits.

Systems such as those depicted are erasable either in bulk orselectively. Such erasure may be accomplished by uniform irradiation,again, utilizing a wavelength of 0.65 micrometers or less, or bynonuniform irradiation defining a negative image at such angle ofincidence as to correspond with a selected positive image to be erased.

2. The Composition The fundamental composition of materials herein,while designated by the nominal formula LiNbO,, may and usually doesvary somewhat from the indicated stoichiometry. It is well known thatsuch variation may be desirable, for example, to expedite growth inwhich instance the congruent 0.486more nearly coinciding with theformula (Li,0) (Nb,O is indicated. in essence, the improvement resultingfrom the invention involves addition of iron ions to the composition.The mechanism responsible for the refractive index change is one ofelectron hopping, as discussed in section 4 herein. Specifically, theresponsible mechanism involves liberation of an electron by alight-energized Fe ion. Electrons so liberated are captured by randomlysaturated Fe ions. The significant index change corresponds with thechange in concentration of Fe ions in selected regions.

It is seen, therefore, that the inventive teaching may take the form ofa prescribed Fe content, of course, always assuming at least an equalnumber of Fe ions to act as capture centers. From the standpoint of theinvention, it is prescribed that iron be added to the composition so asto result in at least 50 ppm of Fe (Fe' cations relative to lithium plusniobium cations). It is, therefore, required that iron content alsoinclude at least 50 ppm of Fe. The total minimum iron content to beadjusted by addition, in accordance with the invention, is, therefore,at least 100 ppm total iron, assuming half the total iron to be Fe orsuch total amount of iron inclusion as includes 50 ppm Fe". This minimumcontent has been found necessary to result in a significant improvementin diffraction efficiency as compared with LiNb093 crystals grownconventionally.

A maximum Fe content on the same basis of 500 ppm is prescribed with aview to the transparency necessary for media having a thickness of theorder of 0.25 mm (a thickness considered minimal for volume storage ofmultiple images in apparatus such as that depicted in the FIGURE.

Fe has little effect on transparency within the normal transparencyrange of LiNbO, (0.35 micrometer- 7.0 micrometers). For the mosteffective utilization, it is again desired that there be at least 500ppm of Fe corresponding with the maximum indicated Fe content. There is,however, little disadvantage in increasing the Fe content even to levelsseveral multiples of this amount.

A preferred minimum of about 100 ppm Fe (and therefore also at leastabout 100 ppm Fe) is based on the observation that such content resultsin a diffraction efficiency of the order of 25 percent in a 2 mm thickcrystal representing a 100 times improvement over usual optical gradeLiNb0 3. Processing Growth of LiNbt), for the inventive purposes iscarried out in the usual manner. The technique usually employed for thebest optical grade material is seeded growth by a Czochralski technique.Starting ingredients, usually forming a congruent composition, may be LiCO and Nb,o,. The procedure is described in Vol. 48, Journal of AmericanCeramic Society, p. 112, (I965) and usually consists of grinding ofinitial ingredients by ball milling followed by a series of sinteringand regrinding steps until complete reaction is obtained. After reactionthe product is melted and the crystal is formed by pulling.

lron addition may be made at any time during processing. For expediency,such addition is generally made to the initial ingredients. Suchingredients of a purity considered acceptable for optical devicesgenerally contain of the order of 1 ppm total iron. It is generallydesirable to continue to use ingredients considered acceptable foroptical grade material since the inventive device, like other opticaldevices, is desirably kept reasonably free of uncontrolled scattering orabsorption centers. Accordingly, the amount of iron to be added at thisstage is of the order of 100 ppm based, again, on the total cationcontent of the starting ingredients. Such iron addition may be made inthe form of Fe,0,, FeO or metallic Fe. Uniform dispersion is assured byfollowing the usual grinding and sintering procedures.

Actual growth may be carried out utilizing standard furnace equipmentand standard ancillary apparatus such as supports, crucibles, etc. Somematerials incorporated in standard apparatus may contribute as much asan additional ppm of iron. Such addition is of little consequence fromthe standpoint of the invention.

After growth, it is necessary to ferroelectrically pole the finalcrystalline material to eliminate domain walls. This is accomplished,again, in the usual manner by maintaining a current of the order of 5milliamps per square centimeter of crystal area through the crystalordinarily while cooling the crystal from near its melting point over aninterval of about 50 C. This poling procedure is necessarily carried outover a temperature range which includes the Curie point; and this Curiepoint, depending on compositional variations encompassed within theformula (Li,O) (Nb,O may vary over the temperature range of from l,050to l,l80 C. To avoid clumping (i.e., inhomogeneous distribution) of ironions, it is desirable to keep poling time to a minimum. In general,poling may be accomplished in a period of 5 minutes or less.

The stability of Fe relative to Fe increases with increasing temperatureand the as-grown material has a thermodynamic distribution whichprovides for the maximum Fif /Fe ratio. It has been found that annealingat temperatures substantially below the Curie temperature, results insome decrease in diffraction efficiency. Accordingly, it is deduced thatthe Fe' 'lFe ratio is reduced to less than 50 percent by such treatment.This effect is particularly noticeable for annealing below about 700 Cand so such annealing is generally to be avoided. If other desideratarequire annealing below 700 C, the Fe/Fe ratio may be increased to amore desirable level by subsequent high temperature annealing.

4. Mechanism Explanation of refractive index change in lithium niobateis dependent on a valence change of iron ions during device operation.It is inherent in holographic recording that the electromagneticradiation corresponding with interference fringing is of nonuniformintensity across the crystal. The effect is to preferentially excite theFe ions located in the most intensely illuminated regions of thecrystal. Such excited ions may then liberate electrons, so beingconverted to Fe. Electrons so liberated are captured by other Fe ionsgenerally in less intensely illuminated regions of the crystal, and suchions, in turn, are converted to Fe". The overall effect is a changingdistribution of Fe ions which results in the creation and redistributionof local refractive index inhomogeneities via the electronic effect.

5. Examples The following examples recite different procedures utilizedfor growing designated compositions. In Examples 2 and 4, iron is addedand processing is such as to assure a reasonable level of Fe. Example 3is added to show the effect of low temperature annealing. Example 1,included as a reference, sets forth the usual growth procedure. Eachexample includes measured diffraction etfciency. The diffractionefficiency value set forth in each instance is a "saturation value,"i.e., such measurement is made subsequent to sufficient exposure toresult in the maximum index change attainable under the given operatingconditions. The test source used for the measurements produced anintensity of about 10 watts per square centimeter and had a wavelengthof 5,!45 angstroms. For comparison purposes, all sections utilized inthe examples were 0.2 cm in the thickness direction. In each instance acalculated value of Fe ion content is set forth.

EXAMPLE 1 A crystalline section cut from a congruent compositionproduced from starting materials having a total iron content of about 1ppm and utilizing standard grinding and sintering under conditions suchthat iron content was increased to about 10 ppm in the final crystal wasutilized. Exposure of such crystal to a beam intensity of 10 watts persquare centimeter at room temperature resulted in a difi'ractionefficiency of 0.2 percent in a period of 1 minute. The Fe concentrationof the sample was of the order of 5 ppm.

EXAMPLE 2 A crystalline sample was prepared from the same startingingredients of those of Example 1, with the addition of 0.05 molepercent Fe,0,. Such section incorporated in a device operating on thegeneral principle of that of FIG. 1, resulted in a diffractionefficiency of 44 percent in less than 1 minute. The Fe concentration ofthis crystal was about 250 ppm.

EXAMPLE 3 The sample of Example 2 was annealed in air at 700 C for 75hours at 600 C. Such section subjected to the same device environment asthat of Examples 1 and 2 resulted in a diffraction efficiency of 1percent in less than 1 minute. The Fe concentration of the sample was ofthe order of 20 ppm.

EXAMPLE 4 said body having a thickness suflicient to permit storage ofmultiple images therein when irradiated on a given surface by aninformation-containing beam incident on the said surface at any of anumber of angles of incidence corresponding with the number of multipleimages together with means for directing at least a first beam ofcoherent electromagnetic energy at the said crystal, said meansincluding provision for making the said beam incident on the said bodyat any of a number of prescribed angles of incidence, characterized inthat the said material contains iron added in an amount sufficient toproduce a total Fe ion content of at least 50 ppm based on totalcationic content of the said material and at least an equal number of Feions.

2. Arrangement of claim I in which the Fe" content is at least ppm.

3. Storage arrangement of claim 1 in which the means for providing aprescribed angle of incidence consists essentially of an angulardeflector element juxtapositioned to the said crystal.

4. Arrangement of claim 3 in which the said deflector depends for itsoperation upon an acoustooptic interaction with the said beam.

5. Arrangement of claim 1 including 1: and y deflector elements forselectively irradiating discrete areas of the said crystal.

6. Arrangement of claim 5 in which the said deflectors depend for theiroperation upon an acoustooptic interaction.

7. Arrangement of claim 5 in which the said deflectors depend for theiroperation on an electrooptic interaction.

8. Arrangement of claim 7 in which the said deflectors depend for theiroperation on a magnetooptic in- "ffifi'fimms comprising a singlecrystalline body of material within the range designated by the formulaprovided with first means for illuminating selected portions of saidmedium so as to alter the refractive index of said portions and secondmeans for detecting the said change in the refractive index,characterized in that the said material contains iron added in an amountsufficient to produce a total Fe ion content of at least 50 ppm based ontotal cationic content of the said material and at least an equal numberof Fe ions.

I f t i l

1. A volume hologram storage arrangement comprising a single crystallinebody of material within the range designated by the formula(LiO2)0.44-0.5(Nb2O5)0.56-0.5, said body having a thickness sufficientto permit storage of multiple images therein when irradiated on a givensurface by an information-containing beam incident on the said surfaceat any of a number of angles of incidence corresponding with the numberof multiple images together with means for directing at least a firstbeam of coherent electromagnetic energy at the said crystal, said meansincluding provision for making the said beam incident on the said bodyat any of a number of prescribed angles of incidence, characterized inthat the said material contains iron added in an amount sufficient toproduce a total Fe2 ion content of at least 50 ppm based on totalcationic content of the said material and at least an equal number ofFe3 ions.
 2. Arrangement of claim 1 in which the Fe2 content is at least100 ppm.
 3. Storage arrangement of claim 1 in which the means forproviding a prescribed angle of incidence consists essentially of anangular deflector element juxtapositioned to the said crystal. 4.Arrangement of claim 3 in which the said deflector depends for itsoperation upon an acoustooptic interaction with the said beam. 5.Arrangement of claim 1 including x and y deflector elements forselectively irradiating discrete areas of the said crystal. 6.Arrangement of claim 5 in which the said deflectors depend for theiroperation upon an acoustooptic interaction.
 7. Arrangement of claim 5 inwhich the said deflectors depend for their operation on an electroopticinteraction.
 8. Arrangement of claim 7 in which the said deflectorsdepend for their operation on a magnetooptic interaction.